In electrolysis of an aqueous alkali halide solution, particularly sodium chloride, reduction of electrolytic voltage has been an important technical subject from the viewpoint of energy-saving and so forth.
The electrolytic voltage is represented by the sum of voltage increments due to elements present between the anode and the cathode, constituting an electrolytic cell. Thus, a reduction of electrolytic voltage can be attained by analyzing each constitutive element and reducing the voltage arising as a result of its presence. In electrolysis by an ion exchange membrane process, main constitutive elements which may be reduced in terms of voltage increments include electrode overvoltage, liquid resistance between electrodes, membrane resistance, and unevenness of current distribution in the membrane.
Recently, various techiques have been proposed according to the so-called SPE electrolytic process utilizing an electrolytic cell in which electrode catalytic substances are held directly on solid electrolytic membranes (see, for example, Japanese Patent Application (OPI) Nos. 102278/78, corresponding to U.S. Pat. No. 4,197,178; 93690/79, corresponding to U.S. Pat. Nos. 4,210,501 and 4,224,121; and 107493/79, corresponding to U.S. Pat. No. 4,191,618). (The term "OPI" as used herein means a "published unexamined Japanese patent application".) This process is superior particularly in that since there is no electrolytic solution between the electrodes, the generated increase of voltage due to the effect of gas bubbles mainly responsible for the liquid resistance can be substantially eliminated, and it is said to greatly solve the above-described subject. However, many technical problems arise in applying the process to industrial scale electrolysis requiring a large-sized membrane.
In order to overcome these problems, Japanese Patent Application (OPI) No. 93883/81 (corresponding to published European Patent Application No. 0031660) discloses an improved SPE electrolytic process utilizing a newly-developed electrolytic cell in which an electrode member prepared by coating a gas permeable, electrically conductive substrate with a porous electrode catalyst layer is brought into close contact with the surface of a solid electrolytic membrane, and furthermore, a coating layer of low activity substance is provided between the membrane and the electrode catalyst layer to decrease the electrolytic membrane, and furthermore, a coating layer of low activity substance is provided between the membrane and the electrode catalyst layer to decrease the electrolytic voltage and improve the electrolytic performance. Similar techniques of decreasing the electrolytic voltage by providing a layer of an inert substance between the ion exchange membrane and electrode are disclosed in Japanese Patent Application (OPI) Nos. 112487/81 and 23076/82.
However, the effect of decreasing electrolytic voltage to be attained only by providing these low activity substance layers on an ion exchange membrane is still insufficient. It has further been revealed that in operating an electrolytic cell for long periods of time, the voltage increased gradually, and it is therefore difficult to perform a low voltage operation in a stable manner over long periods of time.